Various kinds of image processing apparatuses are known. In recent years, there has been an image processing apparatus capable of processing a double-sided document having images printed on both sides thereof in consideration of environment. When information written in a double-sided document is filed in a double-sided copying machine or a double-sided electronic filing apparatus, the document is read by an image reader such as an image scanner.
In order to compensate the function of reading only either side of a document in a simple image reader, the document is turned over by an operator after the obverse of the document is read, and then, the reverse of the document is read. Otherwise, in a simple double-sided image reader, a document is turned over by a mechanical mechanism after the obverse of the document is read, and subsequently, the reverse of the document is read.
However, the image reader which reads only either side of a document places a significant burden on a user, and further, inconveniently takes much time to read the document. In the meantime, in the double-sided image reader having a mechanical turning-over function, there may accidentally occur drawbacks caused by the mechanical mechanism, for example, paper jamming in turning-over, or mechanical failures. In order to eliminate such inconveniences, there has been known an image processing apparatus including respective readers for both of the obverse and the reverse of a document, in which the readers simultaneously read images of the obverse and the reverse of the document. The use of such an image reader can achieve image processing of double-sided copying or the like at a high speed with few failures.
Now, a digital combined machine having a double-sided copying function will be explained as one example of conventional image processing apparatuses for performing double-sided reading. FIG. 15 is a block diagram illustrating one example of the configuration of a conventional double-sided copying machine. As illustrated in FIG. 15, the digital combined machine comprises a series of constituent units such as a reading unit 1501, an image processing unit 1502, a video controller 1503 and a writing unit 1504; component elements constituting a copying machine (i.e., the section of a copying machine) consisting of a memory control unit 1505 and a memory module 1506; a process controller 1511, a RAM 1512 and a ROM 1513; and various units such as a facsimile control unit 1512, a printer control unit 1513 and a scanner control unit 1514 which are additionally connected via a motherboard 1511.
The motherboard 1511 is constituted of an obverse image transfer bus 1515a for transferring obverse image data and a reverse image transfer bus 1515b for transferring reverse image data. These buses are normally required to transfer image data, which are simultaneously read by the reading unit 1501, at the same timing with the same data structure. Furthermore, the two image transfer buses are required also to input the image data on both sides of a document read by the scanner control unit 1514 installed externally.
The reading unit 1501 is constituted of an obverse reading unit 1501a for reading the obverse of the document and a reverse reading unit 1501b for reading the reverse of the document. In the same manner, the image processing unit 1502 is constituted of an obverse image processing unit 1502a and a reverse image processing unit 1502b. Furthermore, the video controller 1503 is constituted of an obverse video controller 1503a and a reverse video controller 1503b. 
When both sides of the document are read in the image processing apparatus, the volume of the image data input at the same time is twice in comparison with that in the image processing apparatus which reads only either side of the document. Consequently, the read image data is compressed such that the image data is efficiently stored in the memory module 1506 or the image data is efficiently transferred via the various buses.
How the image data is compressed will be explained here. FIG. 16 is a block diagram illustrating one example of the configuration of a data compressor in the memory control unit 1505; and FIG. 17 is a timing chart illustrating the processing timings thereof.
In FIG. 16, the data compressor 1601 comprises: an obverse storage 1602a and a reverse storage 1602b for storing therein obverse image data and reverse image data, respectively; an obverse compressor 1603a and a reverse compressor 1603b for compressing the obverse image data and the reverse image data, respectively; and a controller 1604 for controlling the obverse storage 1602a, the reverse storage 1602b, the obverse compressor 1603a and the reverse compressor 1603b. 
The alphabetic subscripts a and b will hereinafter designate constituent elements relevant to the obverse image data and the reverse image data, respectively, wherein they are not attached when they need not be specially distinguished from each other.
The storage 1602 includes a line memory group 1605 consisting of a plurality of 1-port FIFO memories FM1, FM2, FM3, FM4 and FM5; an output switch 1606 for switching the output destination of the image data; and an input switch 1607 for switching the input source of the image data between the FIFO memories FM1 and FM2.
Incidentally, for the sake of simple explanation, a region to be compressed by the compressor 1603 is assumed to be a rectangular region consisting of four pixels per line multiplied by four lines, that is, four pixels in a main scanning (pixel) direction and four lines in a sub scanning (line) direction, as illustrated in FIG. 18.
As illustrated in FIG. 17, with respect to the compression of image data on the obverse, image data on a first line in the rectangular region of the obverse is written in the FIFO memory FM1a. Next, image data on a second line is written in the FIFO memory FM3a; image data on a third line is written in the FIFO memory FM4a; and image data on a fourth line is written in the FIFO memory FM5a, in sequence. The image data at this time is divided by the obverse output switch 1606a under the control of the controller 1604.
In the stage in which the image data is written in the FIFO memory FM5a, there appear all of the four lines to be compressed by the obverse compressor 1603a. Subsequently, the image data on the first to fourth lines stored in the FIFO memories FM1a, FM3a, FM4a and FM5a, respectively are read out, to be transmitted to the obverse compressor 1603a. This transmission is controlled by the controller 1604. The obverse compressor 1603a compresses the image data on the four lines as a single unit, and then, outputs the compressed image data. The compressed image data is then stored in the memory module 1506.
In the meantime, after the image data on the fourth line is written in the FIFO memory FM5a, image data on a first line in a next rectangular region (i.e., image data on a fifth line) of the obverse is input. The controller 1604 controls to write the image data on the fifth line in the FIFO memory FM2a in order to avoid a memory conflict.
Thereafter, image data on a sixth line is written in the FIFO memory FM3a; image data on a seventh line is written in the FIFO memory FM4a; and image data on an eighth line is written in the FIFO memory FM5a, in sequence.
The controller 1604 reads the image data on the fifth to eighth lines stored in the FIFO memories FM2a to FM5a, respectively, to thus transmit them to the obverse compressor 1603a, while controlling to write image data on a ninth line in a next rectangular region in the FIFO memory FM1a. A series of sequentially input image data on the obverse can be compressed without any hitch by repeating the above-described processing.
In the meanwhile, the image data on the reverse of the document also is input into the data compressor 1601 together with the image data on the obverse. Since the processing of the image data on the reverse is the same as that of the image data on the obverse, the explanation thereof will be omitted here. Compressing operation is repeated every four lines under the control of the controller 1604, so that a series of sequentially input image data on the reverse can be compressed without any hitch.
In this way, since double processing is required in comparison with the processing of the data on either one side when the image data on both sides of the document are input, the data compressing processing has become an important factor for the performance of the apparatus including ease of use of the apparatus. In other words, the conventional image processing apparatus for performing double-sided reading is provided with two compressors for the obverse image data and the reverse image data, for inputting the image data on the obverse and the reverse at a high speed and efficiently transferring and storing the data.
Furthermore, as an apparatus for performing double-sided reading has been devised an apparatus for effectively using a reverse image processing unit block in reading one side of a document which has an image only at the obverse (“an image reader” disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-336396).
However, the conventional image processing apparatus for performing double-sided reading at the same time must be provided with two compressors and two data buses for the obverse and the reverse. Thus, there has arisen a problem of the unwieldy size of a processing circuit.
In particular, in a digital combined machine in which functional units are provided independently of each other and they can be replaced when the function is enhanced, the size of the machine need be reduced as possible in view of the configuration of the machine.